It is often desired to provide a relatively high drive signals to control certain types of devices, for example a hard disk drive head actuator. In the state of the art electronic circuits utilized to move reading and writing heads in disk memory system a voice coil motor is widely used as an actuator to move and position the reading and writing heads. Recently, so-called milli-motors, or milli-actuators, have been considered to provide better, or more accurate, position control of the head. A milli-actuator is generally constructed with a piezo element carried by the positionable arm and to which the head is mounted. A current is selectively applied to the piezo element, which causes the piezo element to deflect, thereby moving the head a small, controllable amount. This provides a fine adjustment to the position of the head. As track densities become more dense, control of the position of the head becomes more critical. Thus, piezo-based milli-actuators are becoming of increasing importance in the head positioning mechanisms.
Although such control circuits are relatively easy to build, they have several problems. Firstly, relatively high drive signals, for example those used to operate piezo elements, may require relatively large circuit components which complicate the overall circuit design and may limit integration. Secondly, most systems are require to be linear, in the disk drive example, an amount the piezo element deflects should be directly proportional to the value of the current applied.
Previously developed amplifier output circuits have addressed some of these problems. For example, output circuits commonly referred to as class A circuits provide low output distortion. Unfortunately, class A circuits inherently consume large amounts of quiescent current. A second class of output circuits is referred to as class B circuits. These circuits consume very little quiescent current. However, class B circuits exhibit substantial crossover distortion. A hybrid of the class A and Class B output circuits is commonly referred to as class AB output circuits. Class AB circuits consume less quiescent current than equivalent class A circuits and they exhibit less crossover distortion than class B circuits. Ideally, a high-performance actuator circuit should be provided in integrated circuit form, and should feature rail-to-rail outputs that impart the maximum voltage with an output stage which minimize cross-over distortion without consuming large amounts of quiescent current. Further, these circuit must include not only high voltage circuitry but also circuitry for use in low voltage input applications. Conventional approaches would use separate stages or dedicated circuitries which adds circuit complexity and/or circuit size.
What is needed is a solution for those applications which require an amplifier with an output stage that operates in class AB and delivers high output voltages and currents in which the inputs stage and output stage are coupled to different supply voltages and which require an efficient silicon area.